V4L2（video for linux version 2），是内核中视频设备的驱动框架，为上层访问视频设备提供统一接口。
V4L2整体框架如下图：
图中主要包括两层和三个结构体：
两层是：
1. v4l2驱动核心层：包含video_device和v4l2_device的分配、设置和注册
2. v4l2下层接口层：具体的底层传感器驱动，现在的摄像头可能支持硬件解码，这就需要在摄像头驱动下面提供解码器IC驱动
三个结构体是：
1. v4l2_device：
struct v4l2_device { /* dev->driver_data points to this struct. Note: dev might be NULL if there is no parent device as is the case with e.g. ISA devices. */ struct device *dev; /* used to keep track of the registered subdevs */ struct list_head subdevs; /* lock this struct; can be used by the driver as well if this struct is embedded into a larger struct. */ spinlock_t lock; /* unique device name, by default the driver name + bus ID */ char name[V4L2_DEVICE_NAME_SIZE]; /* notify callback called by some sub-devices. */ void (*notify)(struct v4l2_subdev *sd, unsigned int notification, void *arg); /* The control handler. May be NULL. */ struct v4l2_ctrl_handler *ctrl_handler; /* Device's priority state */ struct v4l2_prio_state prio; /* BKL replacement mutex. Temporary solution only. */ struct mutex ioctl_lock; /* Keep track of the references to this struct. */ struct kref ref; /* Release function that is called when the ref count goes to 0. */ void (*release)(struct v4l2_device *v4l2_dev); };